If you’ve imported your blog from Blogger and find that all of your Labels turn up as Categories, when you were using them as if they were tags back when you were using Blogger, here’s how to batch-convert (convert more than one at once) them to Tags.
In your wordpress admin, go to Tools and click Import. Then in that list, click Categories and Tags Converter, which “Converts existing categories to tags or tags to categories, selectively”. Then it will show you a list of all your Categories and allow you to convert them.
This tool was particularily helpful on this blog when moving it from our old site on blogger.